Methods of nitriding or carbonitriding steel for the formation of a nitrided layer on their surface which have been so far employed for the purpose of improving their mechanical properties, such as wear resistance, corrosion resistance and fatigue strength, include the following, among others:
(a) The method using a molten cyanate or cyanide salt, such as NaCNO or KCN (Tufftride method); PA1 (b) The glow discharge nitriding method (plasma nitriding method ); and PA1 (c) The method using ammonia or a mixed gas containing ammonia and a carbon source, for example RX gas (gas nitriding or gas soft nitriding method). PA1 (A) holding steel in a gas atmosphere containing fluorine compound gas or fluorine gas and also containing air equivalent to 0.5 to 20 volume % of the total or oxygen gas equivalent to 0.1 to 4 volume % of the total with heating; PA1 (B) after holding steel in a gas atmosphere containing fluorine compound gas or fluorine gas with heating, holding steel in a gas atmosphere containing air equivalent of 0.5 to 20 volume % of the total or oxygen gas equivalent to 0.1 to 4 volume % of the total with heating; or PA1 (C) after holding steel in a gas atmosphere containing air equivalent to 0.5 to 100 volume % of the total or oxygen gas equivalent to 0.1 to 20 volume % of the total with heating, holding steel in a gas atmosphere containing fluorine compound gas or fluorine gas with heating. PA1 1 heating steel in a mixed gas containing fluorine- or fluoride-containing gas and air or oxygen; PA1 2 after heating steel under a fluorine- or fluoride-containing gas atmosphere, introducing air or oxygen with such an inert gas as N.sub.2 into a furnace to hold steel with heating; or PA1 3 prior to the introduction of fluorine- or fluoride-containing gas, introducing air or oxygen with such an inert gas as N.sub.2 into a furnace so as to hold steel with heating, and introducing fluorine- or fluoride-containing gas thereinto where steel is held with heating. As a result, 1 activated fluorine atoms act on the steel surface so as to remove inorganic and organic contaminants therefrom, 2 at the same time, an oxide film on the surface is converted to a fluoride film, which is formed on the steel surface layer of steel to protect thereof, and 3 then, because the fluoride film is removed when nitriding and activated steel base is formed, the effects of fluorine compound gas and the like that quick and uniform penetration and diffusion of nitrogen on the activated surface of the steel base allow to form a good nitrided layer thereon in nitriding, are encouraged by the air or oxygen. Namely, fluorine- or fluoride-containing gas and air or oxygen are used in combination for fluorinating in the present invention. For this reason, the generated fluoride film is reinforced by the O.sub.2 film, which prevents occurrence of uneven nitriding, at the same time, saves consumption of expensive fluorine- or fluoride-containing gas which relates to prevention of uneven nitriding, and in the end, realizes a great deal of cost reduction in nitriding. Therefore, the formation of a low-priced nitride layer can be realized on a broader range of steel types. In addition, the present invention provides a good nitrided layer regardless of types of steel, processing steps, conditions in pre-treatment or the like, and can conduct nitriding even on parts having holes or slits. Furthermore, there are advantages in the invention, for example, nitriding can be carried out on steel types which are difficult to be nitrided such as austenitic stainless steel and all types of heat-resistant steel.
Among these method, (a), which uses hazardous molten salts, has a dark future when evaluated from work environment, waste treatment and other viewpoints. Method (b), which achieves nitriding by means of a glow discharge in an N.sub.2 +H.sub.2 atmosphere under a low degree of vacuum, causes less influences of oxide films owing to some cleaning effect of sputtering but tends to allow occurrence of uneven nitriding due to local temperature differences. In addition, this method is disadvantageous in that articles which can be nitrided are much limited in shape and size and that increases in cost result. Method (c) also has problems, for instance, the treatment process is not very stable but tends to lead to uneven nitriding. Another problem lies in that obtaining a deep nitrided layer requires a fairly long time.
Generally, steel is nitrided at temperatures not lower than 500.degree.C. For the adsorption and diffusion of nitrogen on the steel surface layer, it is desired that the metallic surface should be highly active and free not only of organic and inorganic contaminants but also of any oxide film or adsorption film for O.sub.2. The above-mentioned oxide film, if present, would unfavorably promote dissociation of the nitriding gas ammonia. In practice, however, it is impossible to prevent oxide film formation in gas nitriding. For instance, even in the case of case hardened steel or structural steel whose chromium content is not high, thin oxide films are formed even in an NH.sub.3 or NH.sub.3 +RX atmosphere at temperatures between 400.degree. to 500.degree. C . This tendency becomes more pronounced with steel species containing an element or elements which have high affinity for oxygen, for example chromium, in large amounts.
The oxide formation, such as mentioned above, varies in extent depending on the surface state, processing conditions and other factors even in one and the same work, resulting in unevenly nitrided layer formation. For example, in the typical case of cold worked austenite stainless steel works, satisfactory nitrided layer formation is almost impossible even if passive surface coat layers are completely removed prior to charging into a treatment furnace by cleaning with a hydrofluoric acid-nitric acid mixture. Uneven nitriding occurs not only in gas soft nitriding but also in nitriding of nitriding steel or stainless steel with ammonia alone (gas nitriding). Furthermore, in the case of works complicated in geometry, for example gears, even when they are made of ordinary structural steel, it is a fundamental problem that there is a general tendency to uneven nitriding.
The means or methods so far proposed for solving the above-mentioned essential problems encountered in gas nitriding ad gas soft nitriding include, among others, the one comprising charging vinyl chloride resin into a furnace together with works, the one comprising sprinkling works with CH.sub.3 Cl or the like and heating at 200.degree.-300.degree. C. to thereby cause evolution of HCl and prevent oxide formation and remove oxides therewith, and the one comprising plating works in advance to thereby prevent oxide formation. None of them have been put into practical use, however. Chlorides such as FeCl.sub.2 and FeCl.sub.3 are deposited on the steel surface by HCl, however, these chlorides are very fragile at temperatures below the nitriding temperature and can readily sublime or vaporize, whereby no chloride layer is formed. Furthermore, the handling of the above-mentioned chlorides and the like is troublesome and furnace material is extremely damaged, although they are effective to some extent in preventing oxide film formation. Thus, none of the methods mentioned above can be said to be practicable.